Electrodeposition of chromium

ABSTRACT

Chromium containing deposits free of cracks and having good hardness are plated by the use of a novel aqueous chromium (III) electrolyte. The electrolyte includes an aquo chromium (III)-thiocyanato complex as a source of chromium (III) cations.

.Iadd.This is a continuation of application Ser. No. 97,572 filed Nov.26, 1979, now abandoned. .Iaddend.

FIELD OF THE INVENTION

The present invention relates to the electrodeposition of chromium andits alloys, to the electrolyte utilized in carrying out the method, andto the articles produced by the method.

DESCRIPTION OF THE PRIOR ART

Most chromium electroplating methods utilize hexavalent chromium (VI)cation and sulfate anions as the predominant ingredients in theelectrolyte. In most instances chromium trioxide CrO₃ is the source ofchromium (VI) cation and sulfuric acid is the source of the sulfateanion. Such chromium plating baths are characterized by low platingefficiency. Additionally, such baths emit chromic acid fumes whichpresent a health hazard, unless carefully controlled.

Normally, trivalent chromium is intentionally excluded from chromiumplating solutions as experience has shown that it provides a gray,nodular deposit which is undesirable. However, some reports of desirableelectrodeposition of chromium from chromium (III) baths has beenreported. In an article by D. J. Levy and W. R. Momyer, appearing atpages 1125-1131, November 1970, PLATING, a study of theelectrodeposition of chromium from chromium (III) complexes dissolved innon-aqueous organic solvents was reported. Electrolytes including bothcationic and anionic chromic complexes in organic amide solvents wereutilized. Included among the chromium complexes studied werehexaquochromium (III) ion, Cr(H₂ O)₆ ⁺³ andthiocyanatopentaamminechromium (III) complex, Cr(NH₃)₅ SCN⁺². The use ofaqueous plating systems or aquo chromium (III) thiocyanato complexes isnot taught or suggested by this reference. A subsequent report on theelectrodeposition of chromium from hexaaminechromium (III) formateutilizing ligands, including thiocyanate, for promoting chromiumdeposition from non-aqueous amide solvents was made by D. J. Levy and W.R. Momyer at pages 1563-1570, October 1971, g. Electrochem, Soc., Vol.118, No. 10. J. E. Bride reported on a proprietary trivalent chromiumplating system at pages 1027-1032, November 1972, PLATING, U. K. Pat.No. 1,144,913 discloses the preparation of chromium from an electrolyteincluding a non-thiocyanate complex of chromium (III) with dipolarorganic compounds and water. U. K. Pat. No. 1,333,714 discloses anaqueous chromium (III) plating bath including an aprotic amido buffer.In this reference thiocyanate is not associated with the system. Of someadditional interest is the article by F. Taylor, at pages 53-56, July1952, METAL FINISHING, which discloses a chromium plating bath utilizingprimarily chromium (VI), but including small amounts of chromium (III).U. K. Pat. No. 1,258,021 is of interest as disclosing a chromium (VI)plating bath including thiocyanic acid or its salts as a promoting agentin the electrolyte. None of the references noted utilizes an aquochromium (III)-thiocyanato complex in an aqueous electrolyte solution.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an aqueouschromium or chromium alloy producing electrolyte in which the source ofchromium (III) cation comprises an aquo chromium (III)-thiocyanatocomplex. The present invention also provides a process ofelectrodepositing chromium or chromium containing alloys comprisingpassing an electroplating current between an anode and a cathode in anaqueous plating solution having a source of chromium (III) cationcomprising an aquo chromium (III)-thiocyanato complex.

The complexes utilized in the present invention are aquo chromium (III)thiocyanato complexes, also referred to as aquo thiocyanatochromate(III) complexes. These complexes are of the general formula:

    (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n )

where n is a whole number from 1 through 6. Of course, as alreadyindicated, chromium in these complexes is of the chromium (III) variety.

A brief study of this formula will make it apparent that depending onwhat number "n" is, the complex may be cationic, neutral or anionic.Complexes having charges of +2, +1, neutral, -1, -2, and -3 may bevariously provided. Complexes of this type are described in INORGANICCHEMISTRY 9, 1028 (1970). In fact, it may be possible that each of thesecomplexes is present in the electrolyte in an equilibrium mixture. Oneconvenient method of preparing these complexes is from chromiumperchlorate and sodium thiocyanate, as described in more detail below.

While, not known with certainty, the following mechanism for the platingprocess is postulated. A cation, such as sodium, is present in thesolution from the disassociation of, for example, the originalthiocyanate salt in the formation of the complex, as described in moredetail below. It is believed that these cations provide the primarymeans of carrying current in the solution. Tiocyanate serves as abridging ligand in the complex. Therefore, complexes containingthiocyanate, and incidentally, also carrying chromium (III) are readilyabsorbed onto the conductive substrate of the cathode. Once present atthe cathode the chromium (III) is reduced by available electrons anddeposited as chromium metal. At the same time, the sodium or othercations carrying plating current are not reduced to form a metallicdeposit. As the aquothiocyanatochromate (III) complexes are variouslycationic, neutral or anionic it is believed that the complexes aretransported to the cathode by diffusion as well as by the action of theplating field on the complexes which are cationic. Consequently,chromium is deposited not only from the cationic complexes but also fromthe anionic and neutral complexes present at the cathode due todiffusion.

It has been noted that during deposition substantially all of thethiocyanate anion is liberated to the solution from the complex,although a small amount of thiocyanate may be codeposited with theelectrodeposited metal.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be practiced under a broad range of platingconditions including substantial variations in electrolyte composition.Suitable plating conditions and compositions include, about:

Chromium (Cr⁺³); 0.03-0.5 M

Thiocyanate (SCN)⁻ 0.05-1.0 M

Boric acid (H₃ BO₃); 50 g/l (saturated)

Current density of cathode surface 20-120 mA/cm²

Cell voltage; 7-15 volts

Temperature; 20°-25° C.

pH; 2-3.5

The solvent of choice is water. In each of the following examples thechromium (III) cation was provided from chromium perchlorate Cr(ClO₄)₃although other sources of trivalent chromium may be utilized. Similarlysodium thiocyanate NaSCN was utilized as a source of thiocyanate aniondue to its availability and high solubility. However, other sources ofthiocyanate can be utilized. While a specific amount of boric acid H₃BO₃ has been indicated, any amount of boric acid which provides asaturated solution can be utilized. The experiments from which the aboveranges were derived generally employed constant current conditions.However, in some instances the potential of the cathode was keptconstant relative to a standard calomel electrode arranged to sample thesolution in the vicinity of the cathode, thus providing potentiostaticconditions.

The plating apparatus in which the previously noted experiments wereconducted included a platinized titanium anode. While not required for asingle or short run, it was found to be necessary for long termoperations of the bath to isolate this anode by means of asemi-permeable barrier, in a sodium perchlorate anolyte solution. In theabsence of such isolation the pH of the electrolyte falls away steadily.It is then found that when the pH value falls below 1.5 plating ceases.In the following examples chromium and its alloys were plated onto abrass cathode in a Hull cell in some instances, while in other instancesit was plated onto copper or metalized glass cathodes in differentplating cell arrangements. Chromium and chromium alloys of up to 0.001inch (0.025 mm) thick was deposited utilizing these techniques.

The chromium and chromium alloys plated by the process of the presentinvention are substantially free of impurities, although detection ofsulfur in the deposits indicates that a small amount of thiocyanate mayhave been codeposited with the metal. The effects of such codepositionmay actually be beneficial as providing sites for stress reduction inthe deposit. In each instance the deposited chromium was observed to bebright and was found to be relatively hard, exhibiting a VickersHardness No. (VHN) of 700. The deposits were uncracked, and hadexcellent corrosion resistance.

In each instance the electrolyte exhibited good throwing power for thecurrent and, as previously indicated operated over a wide range ofconditions with good current efficiency.

In addition to pure chromium plating, solutions containing aquothiocyanato chromate (III) complexes have been used to plate alloys ofchromium. In particular, alloys of cobalt and chromium have been platedfrom a solution containing cobalt cations, while alloys of nickel andchromium have been plated from solutions containing nickel cation.

The invention will now be more particularly described with reference tothe following examples of preferred plating solutions and processescarried out in accordance with the present invention.

EXAMPLE I

Preparation of a plating solution according to the invention comprisedthe initial step of preparing a solution of chromium perchlorateCr(ClO₄)₃ in water. This solution was prepared by adding 150 grams ofsodium dichromate Na₂ Cr₂ O₇ to 485 ml of perchloric acid HClO₄ and 525ml of water. Hydrogen peroxide H₂ O₂ was added in dropwise fashion untilthe solution became deep blue color. When this state was reached thesolution was boiled down to half its volume driving off hydrogenperoxide and leaving the required solution of chromium perchlorate. Thissolution was then diluted to 0.15 M concentration to provide a source oftrivalent chromium for plating.

To prepare the plating electrolyte, 150 ml of the diluted 0.15 Mchromium perchlorate solutions was saturated with boric acid H₃ BO₃.Then solution hydroxide NaOH was added in dropwise fashion to adjust thepH of the solution within the range of 1-2. Two grams of sodiumthiocyanate NaSCN were added to the solution and the resultant mixturewas subsequently heated at 80° C. for 1 hour to produce a platingelectrolyte comprising an equilibrium mixture of aquo chromium (III)thiocyanato complexes. The chromium in the chromium perchlorate solutionis hydrated, and in the form of Cr(H₂ O)₆ ⁻³. The equilibrium mixture ofaquo chromium (III) thiocyanato complexes is produced by the progressivereplacement of the H₂ O groups in the hydrated chromium with SCN-groups.

Following the adjustment of the pH of the solution to 2.5 using NaOH,the concentration of the various constituents of this plating solutionis about as follows:

Cr(III); 0.1 M

NCS⁻ ; 0.2 M

H₃ BO₃ ; 50 g/l

Na⁺ ; 2 M

ClO₄ ⁻ ; 0.5 M

A plating process according to the present invention and employing aboveplating solution was carried out as follows.

The plating solution was introduced into a plating cell having aplatinized titanium anode and a flat surfaced brass cathode having anarea of about 38 cm². The anode was isolated from the plating solutionproper by a cationic selective ion exchange membrane or barrier and wassurrounded by an anolyte of sodium perchlorate at 0.5 M concentration.The pH of the anolyte was 2.

A plating current of density 25 mA/cm² of the cathode surface was passedbetween the anode and the cathode for a time of 15 minutes. The platingcurrent was kept constant throughout this time. The temperature of thesolution during plating was 20° C. A total weight of 0.021 g of chromiumwas found to be deposited.

The deposited chromium appeared bright to the eye and uncracked whenexamined under a microscope. Its hardness was measured to be 700 VHN.Its resistance to corrosion in a high humidity, high sulfur dioxideatmosphere was excellent.

EXAMPLE II

An alloy of cobalt and chromium was plated onto a copper cathode in acell similar to that employed in Example I. The plating solution had thefollowing composition:

CoSO₄.7H₂ O; 0.025 M

Cr(III); 0.1 M

NCS⁻ ; 0.2 M

H₃ BO₃ ; 50 g/l

Na⁺ ; 2 M

ClO₄ ⁻ ; 0.5 M

Plating was carried out under constant current conditions and a currentdensity of 150 mA/cm² of the cathode surface was employed for a time of2 minutes. The solution temperature was 20° C. during plating. A weightof 3 mg of cobalt-chromium alloy was deposited. The composition of thealloy was about 20 (At)% Co-80 (At)% Cr. This alloy was found to bemagnetic with a coercivity of 30 Oe. Corrosion resistance as measuredelectrochemically was excellent.

EXAMPLE III

An alloy of nickel and chromium was also plated onto a cathode in a cellsimilar to that employed in Example I. The plating solution had thefollowing composition:

NiSO₄.6H₂ O; 0.25 M

Cr (III); 0.1 M

NCS⁻ ; 0.2 M

H₃ BO₃ ; 50 g/l

Na⁺ ; 2 M

ClO₄ ⁻ ; 0.5 M

Plating was carried out under constant current conditions at a currentdensity of 150 mA/cm². The solution temperature was 20° C. duringplating. The plating current was applied for a time of 2 Minutes. Theresulting nickel-chromium deposit was observed to be magnetic.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A composition of matter for use in theelectrodeposition of .Iadd.bright .Iaddend.chromium containing depositsconsisting essentially of an aqueous solution of aquo chromium (III)thiocyanato .[.complex.]. .Iadd.complexes .Iaddend.as a source oftrivalent chromium, the concentration of .[.the.]. Cr (III) .Iadd.in thesolution .Iaddend.being in the range of 0.03 to 0.5 M.Iadd., and thethiocyanate concentration in the solution being in the range of 0.05 to1.0 M, wherein the concentration of thiocyanate is higher than theconcentration of Cr (III), and wherein the aquo chromium (III)thiocyanato complexes are an equilibrium mixture of complexes of theformula

    (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)

where n is a range of whole numbers from 1 through 6.Iaddend.. .[.2. Thecomposition of claim 1 wherein the aquo chromium (III) thiocyanatocomplex is an equilibrium mixture of complexes having the formula

    (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)

where n is a whole number from 1 through 6..].
 3. The composition ofclaim .[.2.]. .Iadd.1 or 26 .Iaddend.in which the complexes are preparedby the process of reacting thiocyanate anion with chromium perchlorate.4. The composition of matter of claim 1 .Iadd.or 26 .Iaddend.for platingcobalt-chromium containing alloys in which cobalt cations are present insaid aqueous plating solution.
 5. The composition of matter of claim 1.Iadd.or 26 .Iaddend.for plating nickel-chromium containing alloys inwhich nickel cations are present in said aqueous plating solution. .[.6.The composition of claim 1 in which the thiocyanate concentration is inthe range of 0.05 to 1.0 M..].
 7. The composition of claim .[.6.]..Iadd.1 or 26 .Iaddend.in which the pH of the Aqueous solution is in therange of about 2 to 3.5 .[.and.]..Iadd., .Iaddend.the solution issaturated with boric acid. .Iadd., and the thiocyanate-to-Cr (III)concentrations are in about a 2-to-1 ratio.Iaddend..
 8. A method ofelectrodepositing .Iadd.bright .Iaddend.chromium containing material ona cathode, which comprises passing an electroplating current between an.Iadd.insoluble .Iaddend.anode and said cathode in a Plating solutionconsisting essentially of an aqueous solution of .[.an.]. aquo chromium(III) thiocyanato .[.complex.]. .Iadd.complexes .Iaddend.as a source oftrivalent chromium.Iadd., said solution being prepared by means of timeand temperature equilibration in which the thiocyanate is substitutedinto the chromium complexes, and in which chromium (III) concentrationin the solution is in the range of 0.03 to 0.5 M and the thiocyanateconcentration in the solution is in the range of 0.05 to 1.0 M, theconcentration of thiocyanate being higher than the concentration ofchromium (III).Iaddend..
 9. The method of claim 8 .Iadd.or 28.Iaddend.wherein the aquo chromium (III) thiocyanato complex is anequilibrium mixture of complexes having the formula

    (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)

where n is a .Iadd.range of .Iaddend.whole .[.number.]. .Iadd.numbers.Iaddend.from 1 through
 6. 10. The method of claim .[.9.]. .Iadd.8 or 28.Iaddend.in which complexes are .Iadd.an equilibrium mixture ofcomplexes having the formula

    (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)

where n is a range of whole numbers from 1 through 6, .Iaddend.preparedby the process of reacting thiocyanate anion with chromium perchlorate.11. The method of claim 8 .Iadd.or 28 .Iaddend.for platingcobalt-chromium containing alloys in which cobalt cations are present insaid aqueous plating solution.
 12. The method of claim 8 .Iadd.or 28.Iaddend.for plating nickel-chromium alloys in which nickel cations arepresent in said aqueous plating solution. .[.13. The method of claim 8in which the chromium (III) concentration is in the range of 0.03 to 0.5M and in which the thiocyanate concentrations is in the range of 0.05 to1.0 M..].
 14. The method of claim .[.13.]. .Iadd.8 or 28 .Iaddend.inwhich the pH of the aqueous solution is in the range of about 2 to 3.5and the solution is saturated with boric acid.
 15. The method of claim.[.14.]. .Iadd.8 or 26 .Iaddend.in which .Iadd.the pH of the aqueoussolution is in the range of about 2 to 3.5, the solution is saturatedwith boric acid, .Iaddend.the electroplating current density is in therange of about 20 to 120 mA/cm² of the cathode surface.Iadd., thethiocyanate-to-chromium (III) concentration ratio is about 2-to-1, andthe deposited chromium containing material is of about a Vicker Hardnessnumber 700.Iaddend.. The method of claim 8 .Iadd.or 26 .Iaddend.in whichthe electroplating current density is in the range of about 20 to 120mA/cm² of the cathode surface.
 17. The method of claim 8 .Iadd.or 28.Iaddend.in which the anode is immersed in an anolyte and said anode andanolyte are separated from the plating solution by an ion exchangebarrier.
 18. The process of claim .[.17.]. .Iadd.8 or 26 in which theanode is immersed in an anolyte and said anode and anolyte are separatedfrom the plating solution by an ion exchange barrier, and .Iaddend.inwhich the anolyte is sodium perchlorate.
 19. The method of preparing aplating solution for electroplating of .Iadd.bright .Iaddend.chromiumcomprising the step of reacting a thiocyanate anion with chromiumperchlorate in the presence of boric acid in an aqueous solution havinga pH of 1 to 2 to form an aquo chromium III thiocyanato complex of theformula (H₂ O)_(6-n) Cr(SCN)_(n).sup.(+3-n), .Iadd.in .Iaddend.which nis a .Iadd.range of .Iaddend.whole .[.number.]. .Iadd.numbers.Iaddend.from one through six.Iadd., wherein the chromium in theperchlorate solution is hydrated in the form of Cr1H₂ O)₆ ⁺³, andwherein the cyanato complex is formed by the progressive replacement ofthe H₂ O groups, said replacement being produced by heating saidsolution for one hour at 80° C., the thiocyanate concentration being inthe range of about 0.05 to 1.0 M, and the chromium III concentrationbeing in the range of about 0.03 to 0.5 M, and the concentration ofthiocyanate being higher than the concentration of chromium III.Iaddend..
 20. The invention as defined in claim 19 wherein the source ofthiocyanate anion is sodium thiocyanate.Iadd., and wherein theconcentration ratio of thiocyanate to chromium III is about 2 to1.Iaddend.. .[.21. The invention as defined in claim 19 wherein thechromium in the perchlorate solution is hydrated in the form of Cr(H₂O₆)⁻³..]. .[.22. The invention as defined in claim 21 wherein thecyanato complex is formed by the progressive replacement of the H₂ Ogroups..]. .[.23. The invention as defined in claim 19 wherein thereaction takes place at an elevated temperature..]. . The invention asdefined in claim 19 wherein the reaction solution contains sodiumhydroxide to control the pH. .Iadd.25. The composition of claim 1wherein said equilibrium is produced by heating said composition ofmatter for one hour at 80° C. .Iaddend. .Iadd.26. The method of claim 8wherein said equilibration is produced by heating said solution for onehour at 80° C. .Iaddend.